The general configuration of wheels has been followed over the last hundred years, i.e. a pair of wheels at the front and rear sides, respectively, forming a rectangular configuration. The front pair of wheels controls steering and the rear one is responsible for power transmission. Also, every wheel in the rectangular configuration of wheels can provide supporting function of the vehicle. Take a sedan for example; its front part is generally provided with an engine and handling system, middle part has the compartment for passengers, and rear part is disposed with gasoline tank and luggage compartment.
For a general rear-wheel driving vehicle undergoing a turning process, since rear wheels are fixed without turning, the common center of turning will fall on the co-axis of rear wheels, such that the turning radius is increased.
When a vehicle goes straight without turning, four wheels would have identical rotation speed. When a vehicle with four wheels is turning, however, four wheels must have different rotation speeds; outer wheels must rotate faster than inner ones. Two of four wheels are usually power wheel (driving wheels), and the one in the far end has to rotate faster and the other one rotates slower or it will slide rather than rotate. Therefore, a differential is needed. Apart from the two driving wheels, the other two are free wheels (driven wheels); the one in the far end will be driven to rotate faster and the other one is driven slower. Therefore, no differential is required without the concern of wear due to sliding.
However, a general rear-wheel driving vehicle undergoing a turning process, since rear wheels are fixed without turning, the common center of turning will fall on the co-axis of rear wheels, such that the turning radius is increased.
A double-head vehicle can have an individual engine for each head or a common engine for both heads. Employing two engines can certainly raise cost, but the requirement of transmitting power to front/rear wheels, four-wheel driving mechanism for example, is complicated and therefore may carry high cost if the mode of sharing a common engine is adopted. Regardless of the two-engine or single engine mode, rear and front wheels have both to be power wheels; therefore, a differential is required for both pairs of wheels. Moreover, a coupling mechanism is required to switch between vehicle head and vehicle tail, i.e. vehicle head and vehicle tail changing sides. The mechanism demands the coupling and coordination of transmission and steering system. Consequently, the cost of the two aforementioned modes of a double-head vehicle is high.
To overcome the drawbacks of the aforementioned rear-wheel drive vehicle, the ROC patent No. 474282, Novel Vehicle Four-wheel Drive Steering System, discloses a novel steering system for four-wheel drive vehicles, which during low speed running, front and rear wheels are revolving in opposite phase, and at medium and high speed running, front and rear wheels are revolving in same phase, comprising a front wheel steering mechanism, a steer core mechanism, and a rear wheel steering mechanism. The patent is characterized in that the central rotating shaft transmits steering wheel angle to the rocker of the steer core mechanism to drive the output rod to rotate as the first input; the stepper motor drive the rotary arm worm gear of the steer core mechanism according to vehicle speed so as to rotate the rotary arm as the second input; when the steer core mechanism receives these two inputs, it drives rocking arm, extendable rod, and output rod to generate an output displace to rear wheel steering mechanism to drive the rear wheels to turn. However, there are a number of drawbacks observed in the aforementioned patent. (1) The steering of the front wheels and rear wheels is controlled by a mechanical mechanism, and thus a high cost is involved; (2) the steering of front wheels and rear wheels are not necessary symmetrical; and (3) it requires a vehicle speed sensor to enable front wheels and rear wheels steering in opposite phase at low speed, and front wheels and rear wheels steering in same phase at medium/high speed, leading to an even more complicated structure. Also, this novel four-wheel steering system requires an expensive differential for both front wheels and rear wheels. Moreover, a coupling mechanism is required to switch between vehicle head and vehicle tail, which demands the coupling and coordination of the transmission and steering system, also leading to a high cost. It is therefore necessary to provide new designs to overcome these drawbacks of the aforementioned.